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From: Karl Ratzsch <mail.kfr@gmx.net>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: Plot data using every
Date: Thu, 31 Oct 2019 08:38:25 +0100
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Am 30.10.2019 um 20:29 schrieb s1291:
> On Wednesday, October 30, 2019 at 8:19:30 PM UTC+1, Hans-Bernhard Bröker wrote:
>> Am 30.10.2019 um 15:38 schrieb s1291:
>>
>>> What if I want to plot the points with even x value using 'every' keyword? that is, the points (2, 2.0), (4, 4.0) (6, 6.0).
>>
>> Then you continue reading the documentation of the 'every' option, which
>> you already found.
> 
> Thank you for your reply. 
> I've read the documentation but I don't see how to do that.

Read again ;) (under "help every")

      plot 'file' every {<point_incr>}
                          {:{<block_incr>}
                            {:{<start_point>}
                              {:{<start_block>}
                                {:{<end_point>}
                                  {:<end_block>}}}}}

Your default start_point is zero. You just set it to "1":

    plot dataf every 2::1

Alternative (plot even data, not even point/line numbers)

    plot dataf us ($1 % 2 == 0 ? $1 : NaN):2